1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a laser printer, which transfers a toner image carried on an image bearing member onto a recording material by electrophotography. More particularly, the present invention relates to an image forming apparatus including a transfer belt that transfers and conveys a recording material.
2. Description of the Related Art
In an electrophotographic apparatus in which a recording material is held and conveyed by a transfer belt stretched by a plurality of rollers, the recording material on the transfer belt is electrostatically attracted to the transfer belt when passing through a transfer nip.
However, if the stiffness of the recording material is low, it is difficult to separate the recording material from the transfer belt by utilizing only the stiffness of the recording material and the curvature of a separation roller serving as a separation stretching member that stretches the transfer belt. That is, the recording material remains attached to the transfer belt at the position of the separation roller, and this causes separation failure. Accordingly, Japanese Patent Laid-Open No. 9-15987 discloses a structure for separating a recording material by uniformly forming projections on a surface of a separation roller for stretching a transfer belt so as to undulate the transfer belt at a separating position. While this structure allows the transfer belt to be undulated at the separating position, great tension constantly and locally acts on the transfer belt. As a result, the transfer belt locally wears, and the resistance becomes nonuniform. This makes transfer performance unstable.
Japanese Patent Laid-Open No. 5-119636 discloses a method for deforming a transfer sheet bearing a recording material for separation of the recording material while reducing wear due to deformation. In the method of Japanese Patent Laid-Open No. 5-119636, a roller is provided as a raising member that is movable between a position so as to raise the transfer sheet from an inner side and a position so as not to raise the transfer sheet. In this method, the recording material is separated by raising the transfer sheet by the roller. The transfer sheet is not raised while the recording material is not separated. To separate recording materials of various thicknesses without deforming the transfer sheet more than necessary, a thin recording material is raised by a large raising amount, and a thick recording material is raised by a small raising amount.